The present invention relates to production of calcium sulfate from fluorgyp.
When producing hydrogen fluoride (HF) gas from fluorspar and sulfuric acid in kilns, significant quantities of gypsum are produced. In fact, more pounds of gypsum are produced than pounds of hydrogen fluoride gas. The gypsum from the production of hydrogen fluoride is in the anhydrous form and is referred to as xe2x80x9cfluorgypxe2x80x9d, since it usually contains 1 to 2 percent unreacted calcium fluoride. The fluorgyp from a hydrogen fluoride producing kiln will also contain a low level of unreacted sulfuric acid. The level of the unreacted sulfuric acid depends on the particular equipment used to generate hydrogen fluoride. The level usually ranges from 0.1 to 4 percent.
Aluminum fluoride is typically manufactured in a two-step process. First, hydrogen fluoride gas is generated from fluorspar and sulfuric acid in kilns. Second, the hydrogen fluoride gas is then reacted with alumina trihydrate in a three stage fluidized bed reactor. For every pound of aluminum fluoride produced, 2.73 pounds of fluorgyp are produced.
For hydrogen fluoride and aluminum fluoride production the fluorgyp must be neutralized and sent to a storage area. Very large stacks of fluorgyp are a common sight around hydrogen fluoride and aluminum fluoride plants. At the one aluminum fluoride plant, the fluorgyp is slurried in a water stream and pumped to a large containment area where it is neutralized with excess caustic from a nearby alumina plant. The solids are allowed to settle in the containment area and the water is recirculated back to the aluminum fluoride plant as the carrier medium.
A market that uses large volumes of gypsum is Portland cement. In the production of cement, gypsum is used as a set retarding agent. Without gypsum the cement will harden too fast and not give the construction industry the time needed to pour and finish the concrete in the field. In addition, when cement sets too fast, excess heat is generated which can lower the final strength of the concrete produced. Chemically speaking, a set retarding agent is needed in cement to slow down the hydration of C3A (tricalcium aluminate). The set retarding properties are determined by the solubility of the (SO4)xe2x88x922 ion, which coats the C3A particles.
Historically the cement industry has used calcium sulfate dihydrate in the mineral form because it was available at a low price. However, anhydrous calcium sulfate will also function as a set retarding agent for cement because the solubility of anhydrous calcium sulfate in water is only slightly lower than that of the dihydrate. For Portland type I and II cements, the dihydrate form of gypsum is usually added at a level of approximately 3 to 4 percent.
A principal objective of the present invention is to convert fluorgyp to calcium sulfate useful as a set retarding agent for cement.
An advantage of the present invention is that sulfuric acid in the fluorgyp is neutralized in a solid phase reaction, so that the resulting product is a dry powder rather than a wet cake.